Aluminum ore (“bauxite”) is considered the main source of aluminum. On an industrial scale, bauxite is first processed into aluminium oxide (also known as “aluminium(III) oxide”, “aluminum hydroxide”, “alumina trihydrate” and “alumina”), which is then converted to aluminium metal. The principle means of refining bauxite and producing aluminum hydroxide at the industrial scale is by the well-established method of the Bayer process.
In general, the Bayer process typically comprises: a digestion stage, wherein alumina is extracted by digesting the bauxite ore in a solution of sodium hydroxide solution (“caustic” or “caustic solution”) forming an aqueous sodium aluminate solution; a clarification stage, wherein solid phase residue (“red mud” or “bauxite residue) is removed, via sedimentation and filtering, from the pregnant liquor (supersaturated in sodium aluminate), leaving sodium aluminate in solution; a precipitation stage, wherein aluminum hydroxide is precipitated from the sodium aluminate solution (“liquor” or “Bayer Process liquor”) and grown in the form of aluminum hydroxide crystals (crystallization); a classification stage, wherein crystal seeds are separated from the aluminum hydroxide product material; and then a calcination stage, wherein the aluminium hydroxide decomposes to aluminium oxide, the alumina end product. More detailed descriptions of the Bayer Process and its process steps are readily available. For example, a more detailed, but not comprehensive, description of the Bayer Process step can be found in U.S. Pat. No. 8,298,508, which is herein incorporated by reference in its entirety.
Production of alumina is energy intensive and costly. Despite using the Bayer Process for well over a century, there are still many challenges to improve the process. With lower grade ore, greater mineral complexity and environmental concerns, process optimizations that can maximize product yield, conserve energy, and minimize operational costs are pursued on an ongoing basis. Attempts to meet the targets above are faced with many complicating factors including impurity levels in liquor, caustic embrittlement at higher concentration. Moreover, specific techniques employed in industry for the various steps of the process can vary from plant to plant, making consistent improvements difficult.
Particular areas of focus for process optimization include maximizing liquor productivity/yield and reducing energy usage. This includes the precipitation stage, wherein the precipitated solid aluminum hydroxide is collected as product through the application of multiple precipitation and flocculation steps of the clarified sodium aluminate liquor. Maximizing the output of aluminate crystals during this stage is important in the economic recovery of aluminum values by the Bayer process.
Bayer process operators strive to optimize their crystal formation and precipitation methods so as to produce the greatest possible product yield from the Bayer process while producing crystals of a given particle size distribution. Relatively large particle sizes are beneficial to subsequent processing steps required to recover aluminum metal. Undersized alumina trihydrate crystals, or fines, generally are not used in the production of aluminum metal, but instead are recycled for use as fine particle alumina trihydrate crystal seed. As a consequence, the particle size of the precipitated trihydrate crystals determines whether the material is to be ultimately utilized as product (larger crystals) or as seed (smaller crystals). The classification and capture of the different sized trihydrate particles is therefore an important step in the Bayer process.
This separation or recovery of alumina trihydrate crystals as product in the Bayer process, or for use as precipitation seed, is generally achieved by one of multiple techniques, including one or a combination of settling, cyclones, filtration and/or a combination of these techniques. Coarse particles settle easily, while fine particles settle slowly. Typically, plants will use two or three steps of settling in order to classify the trihydrate particles into different size distributions corresponding to product and seed. In particular, in the final step of classification a settling vessel is often used to capture and settle the fine seed particles. The overflow of the last classification stage is returned to the process as spent liquor to be used back in digestion. Trihydrate particles reporting to the overflow in this final settling stage are typically not utilized within the process for either seed or product. Effectively such material is recirculated within the process, creating inefficiencies.
Particle size of the precipitated trihydrate crystals obtained in the classification step and capture of trihydrate particles, whether the material is to be ultimately utilized as product or as seed, and the minimization of aluminum trihydrate fines in the overflow are direct contributors to the quality and quantity of alumina output. As such, achieving further process efficiencies in this area is an ongoing pursuit.
In efforts to improve the efficiency of the aluminum trihydroxide separation process, certain compounds, including various flocculants, that are soluble or dispersible in the process liquid, such as dextran, a polysaccharide, are added as a process additive. Conventional technology employs the addition of synthetic water soluble polyacrylate flocculants and/or dextran flocculants to enhance settling characteristics of the alumina trihydrate particles in the classification process and thus, reduce the amount of solids in the spent liquor. Cross-linked dextran or cross-linked dihydroxypropyl cellulose are also employed to enhance the settling of fine alumina trihydrate crystals. While such treatments, including flocculant compositions, are often used in the trihydrate classification systems of Bayer plants, some require extensive formulation time and have restricted usage and delivery costs, which altogether negatively impact efficiency and contribute to cost.
Despite the continuous and ongoing development of methods suitable for obtaining aluminum hydroxide crystals with increased particle size, there is still a desire for improvements and enhancements for the aluminium hydroxide production process to address production quality and economic concerns. There is need and utility for methods of and compositions for enhancing particle capture and settling rates, while minimizing the concentration of solids in the overflow after the last stage of classification. Together, these improvements increase process efficiencies, reduce preparation time and material usage, and provide flexibility in application.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR § 1.56(a) exists.